The objective of this study is to identify the mechanisms of dopamine (DA)-mediated regulation of natriuresis in aging. DA is an endogenous natriuretic hormone that regulates salt reabsorption in the kidney proximal tubules by inhibiting apical membrane sodium-hydrogen exchanger-3 (NHE3) and basolateral membrane sodium-potassium ATPase (NKA). Regulated activities of NHE3 and NKA is critical for total body salt, water and mineral homeostasis and therefore contributes significantly to control of blood pressure, cardiac function, nerve impulses, and bone metabolism. We have demonstrated that expression of a sodium-hydrogen regulatory factor (NHERF-1) is essential for DA- mediated regulation of NKA. In cells lacking NHERF expression, DA fails to regulate NKA function. We demonstrated that NHERF-1 associates with NKA, Dopamine 1 receptor (D1 receptors), and PKC?. Upon stimulation with DA, NHERF-1 association with NKA decreased while association with D1 receptor and PKC? increased. Activation of protein kinase A (PKA) and C (PKC) by DA increases NHERF-1 phosphorylation at serine-77. Herein, we propose that phosphorylation of NHERF-1 allows it to leave the complex allowing NKA phosphorylation and endocytosis. Further, we hypothesize that in aging the loss of NHERF-1 - D1R interaction is responsible for salt-sensitive hypertension. We will address this hypothesis in two specific aims. In Aim 1, we will compare NHERF-1 expression and phosphorylation in vehicle and dopamine treated FBN rats. We will determine the association of NHERF-1 to NKA, D1 and D5 receptors, PKA, and PKC?. In Aim 2, we will treat NHERF-1 knock-out animals with high salt or dopamine and determine the importance of NHERF-1 in salt-sensitive hypertension in aging. We anticipate that these studies will provide new insights into the regulation of renal proximal tubular NHE3 and NKA by DA and provide potential therapeutic targets for treatment of disorders associated with abnormal sodium homeostasis in aging.